1. Field of the Invention
The present invention relates to a process for forming a tube having a relatively thick-walled end portion. More particularly, the invention relates to a cold forming process for manufacturing such tubes from tube workpieces.
2. Description of the Prior Art
U.S. Pat. No. 3,837,205 dated Sept. 24, 1974 in the name J. A. Simon discloses a process for forming a tube having a relatively thick-walled end portion in which a tube workpiece is extruded through an annular extrusion orifice defined between a circular extrusion opening and a sizing punch having a stepped surface and positioned centrally within the extrusion opening. In an initial extrusion stage, the workpiece is extruded over a relatively large diameter portion of the central sizing punch defining a thin annular orifice, whereby a thin-walled tube section is extruded. Subsequently, the punch is employed in a retracted position, so that a relatively small diameter portion of the punch extends within the extrusion opening and defines a thicker annular orifice, permitting the thick-walled end portion of the tube to be extruded.
With this process, however, the material of the workpiece needs to be subjected to high pressure to commence and maintain plastic flow, and therefore there is considerable frictional energy loss owing to the reaction between the workpiece and the surfaces of the extrusion die and punch. This energy loss is particularly acute as, in order to achieve extrusion, the blank needs to be completely confined with its surfaces in contact with the extrusion ram, the die cavity, and the punch. There is therefore a large surface area of the extrusion die members in rubbing contact with the plastically flowing material of the blank.